Known in the art is a fuel separation system separating fuel supplied as a stock material (that is, “stock fuel”) to produce fuels differing in properties from the stock fuel. As such a fuel separation system, for example, there is the one provided with a separation membrane selectively passing the high octane value component in the fuel (for example, Japanese Patent Publication (A) No. 2004-232624). In this fuel separation system, high octane value fuel with an octane value higher than the stock fuel is produced at one side of the separation membrane, while low octane value fuel with an octane value lower than the stock fuel is produced at the other side of the separation membrane.
In particular, in the fuel separation system described in Japanese Patent Publication (A) No. 2004-232624, the stock fuel supplied to the separation membrane is raised in temperature by a heat exchanger to increase the ratio of the amount of high octane value component passing through the separation membrane relative to the amount of high octane value component contained in the fuel. Due to this, it is possible to raise the separation efficiency of the stock fuel in the fuel separation system.
However, as explained above, in a fuel separation system, to raise the separation efficiency of the stock fuel, it is necessary to raise the temperature of the stock fuel supplied to the separation membrane. For this reason, in most cases, the fuel separation system is provided with a fuel heating device for heating the stock fuel, in particular, a heat exchanger.
However, if using a heat exchanger for heating the stock fuel, depending on the type of the heat exchanger, sometimes it is not possible to accurately control the amount of heat supplied to the stock fuel and sometimes the fuel ends up being excessively heated by the heat exchanger.
Further, if the internal combustion engine is stopped, usually the feed of fuel to the internal combustion engine is made to stop. If the feed of fuel to the internal combustion engine is stopped, the fuel in the fuel feed passage remains as is at that position. Therefore, at the time when the internal combustion engine is stopped, the fuel which was in the heat exchanger remains as is inside the heat exchanger.
However, in general, a heat exchanger supplies heat to stock fuel in the heat exchanger over a certain period even after the internal combustion engine stops. For this reason, the fuel remaining in the heat exchanger after the internal combustion engine stops will sometimes end up being excessively heated. If fuel ends up being excessively heated in this way, spoilage of the fuel, incomplete circulation of the fuel due to bubbles in the fuel feed passage, etc. may be incurred.